1. Field of the Invention
This invention relates to polymeric capacitors. More particularly, this invention pertains to polymeric capacitors embedded within printed circuit boards or other microelectronic devices. The capacitor comprises a pair of parallel conductive foils separated by a dielectric. Each of the foils has a dielectric layer on its surface, and the dielectric layers are attached to one another. The capacitor exhibits excellent void resistance.
2. Description of the Related Art
A capacitor is a device used for introducing capacitance into a circuit, and functions primarily to store electrical energy, block the flow of direct current, or permit the flow of alternating current. They comprise a dielectric material sandwiched between two electrically conductive metal layers, such as copper foils.
In general, the dielectric material is coupled to the electrically conductive metal layers via an adhesive layer, by lamination, or by vapor deposition. U.S. Pat. No. 5,155,655 describes one method for forming a capacitor wherein a single sheet of a dielectric material is laminated together with two conductive foils. To optimize the performance of a capacitor, it is important that the dielectric material employed has good material properties exhibiting qualities such as superior adhesion, high dielectric strength and good flexibility.
Capacitors are common elements on printed circuit boards and other microelectronic devices. The capacitor is electrically connected either as a discrete element on the circuit board or may be embedded within the circuit board. Of these options, it has been preferred to form printed circuit boards having embedded capacitors to maximize the surface area of the circuit board for other purposes. The capacitance of a capacitor depends primarily on the shape and size of the capacitor layers and the dielectric constant of the insulating material. There are various known types of dielectric materials known in the art. For example, the dielectric material may be a gas, such as air, a vacuum, a liquid, a solid or a combination thereof. Each material has its own particular properties.
In forming capacitors for use in printed circuit boards, a dielectric material such as a glass reinforced polymer matrix has been used. However, the performance of capacitors of this type has been limited by factors such as the limited minimum thickness of the dielectric material, which detracts from the flexibility of the capacitor and attainable capacitance, the effect of bond enhancers on the metal foils, low dielectric constant and poor dielectric strength.
It is desirable to form a capacitor for a circuit board having a high dielectric constant and an extremely thin layer of dielectric material, thus increasing the capacitance and flexibility of the capacitor. However, a common problem frequently associated with such extremely thin dielectric layers is the formation of microscopic voids or other structural defects in the layer. For example, U.S. Pat. No. 5,161,086 provides a capacitor laminate having a single thin sheet of a dielectric material between two sheets of conductive foil. Dielectric layers of this type are highly vulnerable to the formation of voids, and are time consuming to detect and remedy.
The present invention provides a capacitor that solves the problems of the prior art. The capacitors of the present invention comprise a pair of conductive foils, a pair of thin dielectric layers, with one dielectric layer on a surface of each of the foils. The two conductive foils are pressed together such that the dielectric layers are attached to one another forming a single, homogeneous dielectric. By using two thin dielectric layers, any structural voids present in one of the individual dielectric is covered when the other dielectric is adjoined. The likelihood that each of the two dielectric layers has a void congruent with a void on the other dielectric layer is extremely remote. This increases the reliability and physical strength of the capacitor and eliminates a source of manufacturing flaws. Also, the thin dielectric layers allow for higher capacitance, greater heat conductivity and greater flexibility of the capacitor. The insulating dielectric preferably comprises a thermoplastic polymer or a thermosetting polymer having a high dielectric constant, high resistance to thermal stress and low moisture absorption. Together, these factors offer a significant improvement in performance and cost over prior art capacitors and printed circuit boards.
The invention provides a capacitor, which is comprised of a pair of electrically conductive foils, a pair of dielectric layers, one dielectric layer being on a surface of each of the foils and the dielectric layers being attached to one another.
The invention also provides a capacitor which comprises an electrically conductive foil, a first dielectric layer on a surface of the conductive foil, a second dielectric layer on the first dielectric layer and an electrically conductive layer on the second dielectric layer.
The invention still further provides a process for forming a capacitor which comprises applying a first dielectric layer onto a surface of a first conductive foil; applying a second dielectric layer onto a surface of a second conductive foil; and then attaching the first and second dielectric layers to one another. An optional reinforcement sheet maybe inserted between the first and second dielectric layer before attachment.
Also provided are capacitors wherein at least one conductive foil or conductive layer thereof comprises part of an electrical circuit, a chip carrier, a microelectronic device or printed circuit boards comprising the capacitors of the invention.